Ob-: /ob/ pref. Obligatory. A piece of netiquette
acknowledging that the author has been straying from the newsgroup's
charter topic. For example, if a posting in alt.sex is a response
to a part of someone else's posting that has nothing particularly to
do with sex, the author may append `ObSex' (or `Obsex') and toss off
a question or vignette about some unusual erotic act. It is
considered a sign of great winnitude when one's Obs are more
interesting than other people's whole postings.